The present invention relates in general to powersplit hybrid vehicle drive systems, and, more specifically, to operation of the drive after a fault occurs in the connection to or functioning of the battery and/or variable voltage converter.
One type of hybrid electric vehicle powertrain is the powersplit drive system of the type disclosed in U.S. Pat. No. 8,425,377 and U.S. Pat. No. 7,686,723, both incorporated herein by reference. The powersplit drive system has two sources of power. The first source includes an internal combustion engine and the second source is a combination of an electric motor, a generator, and a storage device such as a battery pack. Engine power is divided into two power flow paths at any generator speed and vehicle speed. Engine speed is controlled by the generator, whereby the engine speed can be decoupled from the vehicle speed within the allowed speed range of the generator. This mode of operation is called positive powersplit when the generator is generating electrical power using mechanical power input from the engine.
Because of the mechanical properties of the planetary gearset, the generator can distribute power to the planetary gearset to drive the vehicle. This mode of operation is called “negative powersplit”. The combination of a generator, a motor and a planetary gearset thus can be considered to have electrical continuously variable transmission (e-CVT) characteristics.
A generator brake can be activated so that engine output power is transmitted with a fixed gear ratio to the torque output side of the powertrain through a mechanical path only. The first power source can only produce forward propulsion of the vehicle since there is no reverse gear. The engine requires either generator control or application of a generator brake to transmit output power for forward drive. When the second power source is active, the electric motor draws power from the battery and drives the vehicle independently of the engine for both forward drive and reverse drive. In addition, the generator can draw power from the battery and drive against a one way clutch on the engine power output shaft to propel the vehicle in a forward direction. This mode of operation is called “generator drive mode”. A vehicle system controller coordinates the two power sources so that they work together seamlessly to meet a driver's torque demand without exceeding powertrain system limits. The vehicle system controller allows continuous regulation of engine speed for any given vehicle speed and power request. The mechanical power flow path provides efficient power delivery through the planetary gearset to the driveshaft.
By optimizing operation of the internal combustion engine (ICE) and by implementing regenerative braking, a hybrid electric vehicle with the powersplit architecture significantly improves fuel economy. Because of the large number of additional vehicle components (e.g., variable voltage converter (VVC), motor inverter, generator inverter, battery, motor, and generator), however, overall system cost is increased and additional concerns over vehicle reliability and durability are introduced.
Fault conditions that should be monitored for include electrical faults such as a malfunction of the VVC, the battery, the battery contactors (i.e., relays), or cables which interrupt the flow of electrical power into or out of the battery. Any of these malfunctions would block in ability of the VVC to regulate the voltage level on the high voltage (HV) bus between the VVC and the inverters. Consequently, a significant overvoltage could appear at the HV bus. To prevent such an overvoltage, detection of one of these faults has been used to trigger a shutdown of the vehicle drive system. It would be desirable to obtain a fault-tolerant operation wherein a shutdown of the vehicle drive is avoided during a VVC fault. It is further desirable to achieve such fault-tolerant operation without incurring a significant increase in system cost.